This is all theory, no experience, but what I suspect..
Combo pros: Beautiful stack, the most portable Mesa solution, will look awesome next to my other matching Mesa combo + extension cab (a big bonus of owning Mesa gear is marvelling at their attention to detail and aesthetics), perfect height for access to tone controls (combo on top of extension cab).
Combo cons: Amp will take a lot of vibrations but this may not be an issue with no power tubes in there***no more so than any other head, sitting on any other cab. I've been using these for many years with no vibrational issues yet*** lightweight cab may be flabby, less air volume / speaker makes for a smaller sound***the radiator cabs actually sound quite big. it's still a small cab, but, compared to anything else it's size, it's tuned deep*** a part of me thinks the Radiator cabs will be toylike***Mesa? Toylike?...More like overbuilt*** complicated to mic with hidden 8" speakers etc***what hidden speaker? The radiator? It's not a speaker, it's a port, done differently. It's sued to tune a small box to sound deeper.
Head pros: No vibrations to shake anything loose in there, easy transport to venues that offer in house cabs, Powerhouse 2x12 is probably both bigger sounding and more solid than the radiators, "the visual factor", easy to mic (not that recording bass is easy).
Head cons: 2x12 cab not as portable, the stack would cost more than a Venture 600 combo. And finally but not least.. I simply love the idea of a complete little combo stack, despite how visually uncool it may be for hardrockers like myself***seems wierd, because you'd think they'd sound the same, but the M-Pulse 600 didn't sound at all like the Walkabout head, to me. Not as tubey, not as phat, not as much treble extension***
I guess many of you will say "Hey, the amp is easily removable, so just get the combo and pull out the amp whenever they offer in house cabs!"*** I was pulling the head so often, I just bought a second one to use with other cabs. I also bought a mesa 115 to put under the Scout12 combo, and it does make a really nice stack.***